1. Field of the Invention
The invention relates to a three-phase coil stator in which a plurality of coils are arranged in a ring shape. More particularly, the invention relates to a stator suitable for a stator of a brushless motor.
2. Description of the Related Art
Various motors such as an induction motor and a commutator motor have been conventionally used as the motor. However, in recent years, a brushless motor which has high performance has been frequently used. For example, an inner rotor type brushless motor has a structure such that a stator around which a plurality of coils are wound is formed as an outer stator and a rotor having a magnet arranged in an outer periphery is formed as an inner rotor. The stator is produced by applying a coil winding to a stator core in which a teeth portion for winding the coil is protruded to an inner side in a ring shape. However, because a leading end side becomes narrow between the adjacent teeth portions, the act of coil winding tends to be complicated, and it is not easy to improve a space factor or the coil winding. In order to solve such a problem, there has been invented a stator core in which the stator core can be divided at every tooth portion, and each of the tooth portions can be formed in a linear shape at the time of a coil winding work.
For example, JP-A 2002-58181 discloses a stator in which divided core units are structured by forming a crossover between a plurality of divided cores obtained by dividing a stator core into tooth portion units continuously wound, and the divided core units are coupled in a ring shape in such a manner as to form a rotating magnetic field by three phases of a U phase, a V phase and a W phase. According to this configuration, it is possible to continuously wind by appropriately attaching the divided cores to a jig or the like so as to obtain a layout by which a coil winding work can be easily executed, and it is possible to thereafter couple the divided core units in the ring shape. Therefore, it is possible to make the coil winding work easy and it is possible to improve a space factor of the coil winding.
Further, JP-A 10-271770 (1998) discloses a configuration in that coils are continuously wound every phase by forming a crossover between a plurality of core segments divided per a teeth portion unit, a core segment serial body is formed by coupling concave grooves and convex pieces of the core segments, and a stator is thereafter formed by rounding in an annular body. Also in such a configuration, similarly to the above configuration, it is possible to facilitate the coil winding work and improve the space factor of the coil winding.
Further, JP-A 9-191588 (1997) discloses a configuration such that a stator having a ring shape is formed by continuously winding the coil around a linear unit core configured by integrally coupling a thin coupling portion every teeth portions by connecting between the coils by a crossover, and bending the thin coupling portion after finishing the coil winding. Also in such a configuration, similarly to the above configuration, it is possible to facilitate the coil winding work and improve the space factor of the coil winding.
For the most part, the coil of the stator as described above is continuously wound every phase, and the rotating magnetic field is formed by a coil group of three phases of the U phase, the V phase and the W phase. Accordingly, the number of teeth, that is, the number of slots of the stator is set to an integral multiple of three such as 3, 6, 9 and 12. The slot number of the stator is set in accordance with a design concept of the motor. However, the number of poles of the motor, that is, the number of poles of the magnet in the rotor is important in view of the design of the motor, and the number of poles of the magnet is determined while taking a normal rotation speed and a maximum rotation speed of the motor, and a magnitude of the motor into consideration, and the optimum number of slots of the stator is selected in correspondence to the number of poles of the magnet.
As described above, the number of poles of the magnet and the number of slots are determined on the basis of the design concept. However a description will be given here by exemplifying a brushless motor in which the number of poles of the magnet is 8 or 14, and the number of slots of the stator is 12. In this case, a cogging torque is lowered by setting the number of poles of the magnet to 14 in comparison with the case of 8 poles, and there is an advantage in that a large amount of torque can be obtained at a low rotation speed range such as about 1000 RPM.
FIG. 11 shows a configuration of a brushless motor 900 having 8 poles and 12 slots, and FIG. 12 is a schematic view showing a coil polarity thereof. Magnets 91 disposed in a periphery of a rotor 90 are arranged in 8 poles at a uniform interval in a circumferential direction such that N poles and S poles are alternately formed. On the contrary, 12 coils 93a to 93l of a stator 92 are arranged at a uniform interval in a circumferential direction so as to surround the rotor 90. In the brushless motor 900 having 8 poles and 12 slots as described above, as shown in FIG. 11, the coils 93a, 93d, 93g and 93j are arranged so as to form the U phase, the coils 93b, 93e, 93h and 93k are arranged so as to form the V phase, and the coils 93c, 93f, 93i and 93l are arranged so as to form the W phase. This arrangement is assumed to have the best motor efficiency in the brushless motor 900 having 8 poles and 12 slots. Accordingly, for example, on the assumption that the stator 92 is constituted by 12 divided cores, the U-phase coils 93a, 93d, 93g and 93j are continuously wound around four divided cores by forming a crossover between the respective divided cores, and the V-phase coils 93b, 93e, 93h and 93k and the W-phase coils 93c, 93f, 93i and 93l are continuously wound around respective four divided cores while forming crossovers between the divided cores in the same manner, whereby the respective divided cores are coupled in a ring shape on the basis of an arrangement shown in FIG. 11. In this case, in FIG. 11, a circular mark in each of the coils 93a to 93l denotes a winding start of the coil winding, and a square mark denotes a winding end of the coil winding. Winding directions of the coils continuously wound respectively in the U phase, the V phase and the W phase are the same direction. Further, it is preferable that a wire connection of each of the coils 93a to 93l is constituted by a star wire connection in which four coils in each of the phases are connected in series as shown in FIG. 13. Further, for example, the respective coils 93a, 93d, 93g and 93j of the U phase are in conformity to each other in a phase and a magnitude of induction voltage as shown in FIG. 12, and a circulating current does not flow even by connecting wires in parallel. Therefore, it is possible to form a two-star wire connection of two parallels or a four-star wire connection of four parallels, by appropriately cutting the crossover between the respective phase coils.
On the other hand, FIG. 14 shows a configuration of a brushless motor 901 having fourteen poles and twelve slots, and FIG. 15 is a schematic view showing a coil polarity thereof. Magnets 96 disposed in a periphery of a rotor 95 are arranged in 14 poles at a uniform interval in a circumferential direction such that N poles and S poles are alternately formed. On the contrary, a stator 97 has a structure such that 12 coils 98a to 98l are arranged at a uniform interval in a circumferential direction so as to surround the rotor 95 as in the stator 92. However, a coil polarity is different from the stator 92. In detail, as shown in FIG. 14, the coils 98a, 98b, 98g and 98h are arranged so as to form the U phase in which the coils 98a and 98h and the coils 98b and 98g form unlike poles, the coils 98c, 98d, 98i and 98j are arranged so as to form the V phase in which the coils 98c and 98j and the coils 98d and 98i form unlike poles, and the coils 98e, 98f, 98k and 98l are arranged so as to form the W phase in which the coils 98e and 98l and the coils 98f and 98k form unlike poles. This arrangement is assumed to have the best motor efficiency in the brushless motor 901 having 14 poles and 12 slots. In this case, the unlike poles are expressed by with or without a mark “-” on the U, V and W phases in the figure. In the stator 97, in the respective coils 98a, 98b, 98g and 98h in the U phase, for example, the coils 98a and the coil 98b are not in conformity in the phase and the magnitude of the induction voltage but are shifted at 30 degrees, as shown in FIG. 15. Accordingly, if the coils 98a and the coil 98b are connected in parallel, the circulating current flows and the performance of the brushless motor 901 is lowered. Accordingly, as shown in FIG. 16, the coil 98a and the coil 98b, and the coil 98g and the coil 98h are connected in series, respectively. Because the coils 98a and 98b and the coils 98g and 98h connected in series are in conformity in the phase and the magnitude of the induction voltage, the coils are connected in parallel. Further, the other V phase and W phase are connected in the same manner, and the two-star wire connection as shown in the figure is formed.
In the case where the stator 97 is constituted, for example, by 12 divided cores, the continuous coil winding is executed by forming the crossover between two divided cores while changing the winding direction so as to wind the U-phase coils 98a and 98b connected in series in a clockwise direction around one divided core and to wind them in a counterclockwise direction around the other divided core, the continuous coil winding is executed by forming the crossover between two divided cores while changing the winding direction so as to wind the U-phase coils 98g and 98h connected in series in a clockwise direction around one divided core and to wind them in a counterclockwise direction around the other divided core, and the continuous coil winding is executed by forming the crossover in such a manner that the winding direction is inverted every two divided cores in the same manner as the U-phase, with regard to the V-phase coils 98c, 98d, 98i and 98j, and the W-phase coils 98e, 98f, 98k and 98l, as shown in FIG. 14. Accordingly, the divided cores 99a to 99l are coupled in the ring shape, and neutral points in the respective phases are connected. In this case, a circular mark in each of the coils 98a to 98l denotes a winding start of the coil winding, and a square mark denotes a winding end of the coil winding in FIG. 14.
In the stator 97, a so-called flyer-type or nozzle-type coil winding machine is employed for a coil winding work around the divided core or the like. However, taking into consideration a property of the coil winding machine and a simplification of the coil winding work such as a process of the crossover or the like, it is preferable that the continuous coil winding is executed in the same winding direction. Further, taking into consideration an assembling work of the stator 97 and the simplification of the coil winding work, it is preferable to continuously wind the divided cores or the like as many as possible. However, as exemplified by the brushless motor 901 having 14 poles and 12 slots, if coils of the same phase have the arrangement in which the phase and the magnitude of the induction voltage are different, the coils capable of being connected in parallel are limited. Further, if the coils connected in series have unlike poles, the winding direction of the continuously wound coils is inverted. In such a stator, it is impossible to continuously wind all the divided core or the like of the same phase, and it is impossible to make the winding directions of the continuously wound coils identical.
Further, it is necessary to execute the process of the crossover after forming the crossover between the coils so as to continuously wind around the divided cores or the like, and assembling the divided cores or the like as the stator. Herein, as shown in the stator 97, the crossover between the adjacently arranged coils such as the continuously wound coils 98a and 98b, protrudes in an axial direction of the stator 97 from the coils 98a and 98b, and the size of the brushless motor 901 is enlarged at the space corresponding to the protruding amount of the crossover. However, the layout of the divided cores or the like at a time of assembling is deteriorated and the assembling work is difficult to be executed in the case that the length of the crossover is shortened; therefore, it is impossible to make the crossover too short. Further, the thick coil wire is used in the motor having a high torque; therefore, the crossover becomes thick, and a great force is required for forming the crossover bent in the U shape at the adjacent positions such as the coils 98a and 98b so as to appropriately receive the crossover for space saving, so that it is hard to work by a human power.